I don't know where you live, but if you have a Sally's Beauty Supply or possibly even at WalMart, check out something called "root touch up spray temporary hair color". Here is a link to describe it:

http://www.sallybeauty.com/agebeautiful-...lt,pd.html

This is a spray that lasts until you shampoo it out. I did use it a couple times to tint my own hair on the sides to tone down the gray and match the HP better. You can, of course, spray it on both your own and the HP to get a better match.

My only concern is that you are under such pressure and do not have any time to work out the details of how much to use. Definitely, go lighter with the coats since you can always add more.

Probably, I would test this on your own bio-hair first since it would be easy to shampoo out.

Good luck and be sure to let us know how it turns out.